A. Field of Invention
This invention relates to the removal of contaminants, particularly nitrogen compounds, from aqueous solutions. More specifically, the invention relates to the removal of nitrates from wastewater by biological denitrification.
B. Related Art
A number of methods have been proposed for the denitrification of nitrate-containing aqueous solutions. In certain denitrification systems, wastewater is passed into a denitrification vessel that includes filter media having bacteria or microorganisms on their surfaces. An example of such a denitrification system is shown in U.S. Pat. No. 3,709,364, issued to Savage.
Generally, in such systems, ammonia in the wastewater is oxidized to nitrate-nitrogen, then biologically denitrified in filter media to nitrogen gas and released to the atmosphere. Periodically, the wastewater may be backwashed to remove suspended solids and nitrogen gas bubbles, which adhere to the filter media surfaces.
An important aspect of such systems is the carbon source for the denitrification reaction, which is typically methanol. It has been discovered that more dissolved oxygen in the wastewater results in more methanol or other carbon source being required for the denitrification reaction. Accordingly, there has been a continuing need to control or minimize the dissolved oxygen in the wastewater.
Some have proposed controlling the level of the water in the denitrification vessel. However, level control has a number of shortcomings. For example, when nitrogen release cycles are performed to release accumulated gas, each filter must either be drained of the accumulated water for long periods or face a forced overflow and recycling of large quantities of wastewater. For filters using anthracite coal as a filter medium, the filter boxes must also be drained to a minimum before the backwash can begin to minimize coal loss. Any excess water in the filter tends to increase drain down time and increase the period that the filter is not filtering. However, when level control is not used, dissolved oxygen increases. Accordingly, there exists a need to reduce dissolved oxygen without having to control or maintain the level of the wastewater in the denitrification vessel.